The 3rd Generation Partnership Project (3GPP) is developing protocols for the next generation wireless communication networks (e.g., new radio (NR)). Under the NR, user equipment (UE) is expected to execute applications which perform infrequent small data transmission and/or reception. It is desirable to perform such infrequent small data transmission and/or reception in a power efficient manner. For example, the UE should avoid undergoing radio resource control (RRC) state transition (e.g., from an RRC IDLE state to an RRC CONNECTED state), when the UE only needs to transmit or receive small data.
FIG. 1A shows an RRC state transition diagram under a 4G wireless network, such as long term evolution (LTE), or LTE-Advanced (LTE-A). In an RRC state transition diagram 100, an RRC layer has two states: an RRC CONNECTED state 192 and an RRC IDLE state 194. In FIG. 1A, the RRC state can transition or switch between the RRC CONNECTED state 192 and the RRC IDLE state 194 by using an RRC connection procedure 193 and an RRC release procedure 195.
With reference to FIG. 1B, in the RRC connection procedure 193, an UE 102 in the RRC IDLE state 194 may send an RRC Connection Request message to an evolved UMTS terrestrial radio access network (E-UTRAN) 112, for example, under 4G LTE standard. The RRC Connection Request message is used to request the establishment of an RRC connection. The E-UTRAN 112 may send an RRC Connection Setup message to the UE 102. Upon receiving the RRC Connection Setup message, the UE 102 transitions from the RRC IDLE state 194 to the RRC CONNECTED state 192, and sends an RRC Connection Setup Complete message to the E-UTRAN 112.
With reference to FIG. 1C, in the RRC release procedure 195, the UE 102 in the RRC CONNECTED state 192 may transition to the RRC IDLE state 194 by receiving an RRC Connection Release message from the E-UTRAN 112. For example, the RRC Connection Release message is used to command the release of the RRC connection.
In the 4G wireless network (e.g., LTE, or LTE-A), User Plane (UP) data can be exchanged only in the RRC CONNECTED state 192. Once the UE 102 is in the RRC IDLE state 194, if the UE 102 needs to transmit or receive UP data, the UE must transition to the RRC CONNECTED state 192 regardless of how small the data size is. Moreover, even before the RRC connection procedure 193, there is still a random access procedure that needs to be performed by the UE 102. Thus, if the UE 102 performs the RRC state transition only for small data transmission and/or reception, the latency is high, and the radio resource and power consumption are also high.
In order to reduce latency and power consumption and efficiently allocate radio resources, a new RRC state has been introduced in the NR. The new RRC state is named an RRC INACTIVE state. As shown in FIG. 2, an RRC state transition diagram 200 under the NR includes three states: an RRC CONNECTED state 282, an RRC IDLE state 284, and an RRC INACTIVE state 286. As shown in FIG. 2, the RRC state can transition or switch among the RRC CONNECTED state 282, the RRC IDLE state 284, and the RRC INACTIVE state 286 through various procedures (e.g., procedures a, b, c, d and e). It should be noted that, in RRC state transition diagram 200, a UE in the RRC IDLE state 284 cannot directly transition or switch to the RRC INACTIVE state 286, as has been agreed by the current 3GPP standardization community. Instead, the UE needs to transition to the RRC CONNECTED state 282 through procedure b, then to the RRC INACTIVE state 286 through procedure c.
Different from the 4G wireless network (e.g., LTE, or LTE-A), a UE in the RRC INACTIVE state should incur minimum signalling, minimal power consumption, minimal radio resource costs in the NR, making it possible to maximize the number of UEs utilizing (and benefiting from) this new RRC state. Another key advantage of having the RRC INACTIVE state is that the UE is able to start data transfer with very low delay. For example, the UE may start transmitting or receiving data directly while in the RRC INACTIVE state.
According to the current 3GPP standardization works, after the RRC INACTIVE state is introduced, there have been various discussions on RRC state transitions and related procedures, the UE's mobility is also another aspect that needs to be considered with the RRC state transitions (e.g., procedures c, d and e shown in FIG. 2). For example, since the RRC INACTIVE state is transparent to a core network (CN), the paging procedure while the UE is in the RRC INACTIVE state needs to be performed as RAN-based. It means that once there is a need for the CN to transmit downlink data to the UE, the serving next generation node Bs (gNBs) within a notification area (NA) may transmit the paging messages using notification-based paging, as opposed to by other gNBs or by evolved node Bs (eNBs) within the same tracking area (TA) according to the 4G wireless network based paging procedure (e.g., TA-based paging). The NA under NR is managed by the RAN (Radio Access Network), whereas the TA under 4G wireless network is managed by the CN. In addition, the size of NA will be smaller than the size of TA. Under the RAN-based paging architecture, there is a risk that when the UE, while in the RRC INACTIVE state, leaves a gNB coverage area of the serving gNB and moves to a coverage area of a 4G wireless network (e.g., LTE, or LTE-A) base station (e.g., an eNB).
Thus, there is a need in the art for methods to handle user equipment (UE) RRC state transitions as the UE moves from a NR′ coverage area to a 4G wireless network coverage area.